In data communication systems, data may be transmitted and received at a number of different data rates. The term baud rate is often used to describe a data rate expressed in bits per second (bps). For example, a transmission of data at a rate of 14,400 bps may be referred to as a transmission at 14,400 baud. When the baud rate of an input signal is not known, automatic baud rate detection (autobauding) may be used to determine the baud rate of the input signal.
As shown in FIG. 1, a conventional data communication system includes data terminal equipment (DTE) 110, 120 connected to a communications medium 130 through a modulator/demodulator (modem) 140, 150. It is desirable for a modem 140, 150 to receive transmissions from the DTE 110, 120 at a variety of baud rates. Autobauding, may be used to synchronize the transmission and reception baud rates between a DTE 110, 120 and a modem 140, 150.
A conventional method of autobauding is often performed while the DTE 110, 120 sends commands to the modem 140, 150 in a command mode. For example, when an input signal 200 shown in FIG. 2 includes an AT command of the AT command set, the baud rate of the input signal 200 may be determined using the start bit 210 preceding the "A" character 220 of the AT command. The "A" character 220 shown in FIG. 2 is represented by the ASCII value of 0.times.41 in a least significant bit first format.
A conventional method of autobauding is described by Sauser in U.S. Pat. No. 5,654,983. With reference to FIG. 3, the method of autobauding uses the number of samples between the first transition 330 and second transition 340 of a start bit 210 of an input signal 200. As shown in FIG. 3, the start bit 210 of the input signal 200 is continuously sampled. A first byte of samples 310 and a second byte of samples 320 are shown.
Each byte of samples is compared to a register containing all "1"s to detect a transition of the input signal 200 occurring during that byte. When byte 310 is compared to a register containing all "1"s, the first transition 330 occurring between sample 332 and sample 334 at the beginning of the start bit 210 is detected. The byte 310 is then further analyzed to determine between which samples the first transition 330 occurred. A similar procedure is followed to determine that the second transition 340 at the end of the start bit 210 occurs during the byte 320 and to determine that the second transition 340 occurred between sample 342 and sample 344 of byte 320.
The baud rate of the input signal 209 is calculated based on the number of samples taken during the start bit 210 and based on the known rate of sampling. For example, as shown in FIG. 3, eight samples are taken between the first and second transitions 330, 340 at the beginning and at the end of the start bit 210. If the input signal 200 is sampled at a rate of 8K samples per second, the baud rate of the input signal 200 would be 1K baud.